Municipalities and others recognize that sewer leaks are highly undesirable in that ground water may leak into the sewer and substantially increase the demand upon sewer treatment facilities. There is also the risk of contamination of ground water by liquids leaking out of the sewer. Because of this, it is desirable to inspect sewers for leaks. It is common in such inspection systems to move a TV camera along the sewer to check for leaks and make recordings as to the location of the leaks along the sewer. U.S. Pat. No. 4,107,738 issued to Van Norman discloses a TV camera mounted on skids with an associated hydraulic jet nozzle unit whereby delivery of water under pressure to the jet nozzle unit causes rearwardly-directed jets of water to advance the camera and jet unit along a sewer, with the structure being retracted along the sewer by suitable means, such as rewinding a hose which supplies water under pressure to the jet nozzle unit. This, and other prior art devices that rely on visual inspection require that the interior of the sewer be first flushed clean with water to make a visual inspection possible.
Visual inspection may not always reveal the full extent of defects in the walls of a sewer. In particular, visual inspection only reveals defects manifest on the inside of the sewer. Visual inspection gives no indication of voids or collapsed zones on the outside of sewers that are indicative of leaks from the sewer. These collapsed zones or voids that could be air filled or fluid filled, are indications of leakage of contaminants from the sewer and possible future problems in terms of subsidence of the ground above the sewer. Visual inspection is also not a reliable method of determining the size or shape of the sewer. For example, if due to stresses, the sewer has deviated from a circular cross section, this would not be obvious on a visual inspection. In addition, even where defects are noted on such a visual inspection, repair work is complicated by the necessity of requiring properly surveyed positions for the observed defects. Without this accurate survey, subsequent remedial work becomes harder to carry out.
It is desirable to have a sewer inspection device and method that does not rely only on visual inspection of the sewer. Such a device should preferably be able to detect flaws outside the sewer and changes in the thickness of the sewer. Such a device should be able to make accurate measurements of the location of flaws, whether inside or outside the sewer. Such a device should also be able to function in a sewer filled with a non-transparent fluid. Such a device should also be able to obtain measurements of the size of the sewer. In view of the fact that sewers are commonly filled with explosive gases, such a device should be explosion proof or inherently explosion proof. The present invention satisfies this need.